Note that StreamWriter’s destructor uses m_pStream (perhaps by flushing the stream). This means that the order of destruction is important — StreamWriter must be destroyed before its underlying Stream is.

Now let’s try to write and use some simple managed C++ wrappers for these classes:

As I (gratituously) hinted above, we have a problem due to nondeterminstic destruction. Since the GC does not define the order in which it will destroy managed objects, we cannot guarantee that the ManagedStreamWriter will be destroyed before the ManagedStream. If the ManagedStream is destroyed first, then its Stream will be deleted before the ManagedStreamWriter’s StreamWriter destructor is called. This means that StreamWriter will be using a deleted pointer — a sure recipe for disaster.

I can think of a few possible solutions to this problem:

Have the managed classes implement IDisposable, and require developers to use it to achieve determinstic destruction. The main downside to this approach is “what if developers forget?”

Recapture the interdependencies among unmanaged classes in the managed wrappers. For example, add a reference in ManagedStreamWriter to the ManagedStream object. This will force the GC to properly order their destruction, and is probably the right way to go.

Steven Engelhardt is a Director of Engineering at Relativity, an Adjust Professor of Software Engineering at DePaul University, an avid board gamer, and a father of two. His interests include high-performance data processing systems, server-side and cloud systems architecture, and the Chicago Blackhawks.